Cross-linked rubber members obtained by crosslinking a fluorinated elastic copolymer are excellent in heat resistance, weather resistance, chemical resistance, water resistance and an insulating property and thereby have been widely used for various applications.
Particularly, a tetrafluoroethylene-propylene copolymer is excellent in base resistance and water resistance and used for applications of packings for alkali fluid, engine oil seals, steam resistance packings, packings for pipes for hot water, etc. (for example, Patent Document 1).
Recently, in facilities for producing foods or beverages, food safety is becoming more and more important, and it is strongly demanded to sterilize their production steps. Various sterilization methods such as sterilization with chemicals, sterilization with ozone water and sterilization with ultraviolet ray may be mentioned. Further, sterilization with hot water or steam has been widely used as an effective sterilization method. Further, cross-linked rubber members of fluorinated elastic copolymers have attracted attention as materials which have a durability against all of these sterilization methods.
Further, facilities for producing foods and beverages, facilities for producing pharmaceuticals, medical facilities and facilities for various microorganism tests, in which microorganisms are cultured, are sterilized with hot water or steam in order to avoid contamination of microorganisms.
Further, in industrial facilities, there are many facilities like power generation plants in which hot water or steam is used as a heat medium. Such facilities are more and more required to have the hot water resistance and the steam resistance.
Further, in household facilities, private electric generators or hot-water supply systems are being expanded, and rubber members which are excellent in the hot water resistance and the steam resistance and of which replacement frequency is low are demanded.
In general, as a method for molding a fluorinated elastic copolymer to obtain a cross-linked rubber member, the following method is known. That is, as a case requires, various additives such as a filler, a crosslinking agent and a crosslinking coagent are added, and kneading is carried out by twin rolls, a kneader or an extruder to obtain a fluorinated elastic copolymer composition, followed by molding the fluorinated elastic copolymer composition by e.g. extrusion molding or compression molding with a mold, and at the same time as the molding or after the molding, the fluorinated elastic copolymer composition is crosslinked by means of heating, applying an ionizing radiation or the like to obtain a cross-linked rubber member.
Heretofore, it has been known that when in the above step of kneading or molding, a processing aid such as a higher fatty acid, a metal salt of a higher fatty acid, an aliphatic amine, a natural wax or a polyethylene wax is added to a fluorinated elastic copolymer, such a step can be easily carried out (for example Patent Document 1 or 2). However, it has not been known that such a processing aid remarkably impairs properties, particularly the hot water resistance and the steam resistance, of cross-linked rubber members formed from a fluorinated elastic copolymer.
Particularly, in the above application, in a case where the temperature of hot water or steam to be used is at least 170° C., a cross-linked rubber member of a fluorinated elastic copolymer composition having sufficient heat resistance and steam resistance has not been known.